Conductivity structure of seismogenic region in the western Nagano prefecture

摘要

The magnetotelluric (MT) survey (14 sites)and controlled source audin-magnetotelluric (CSAMT)survey (37 sites) were conducted along NS traversescrossing the concealed fault system which is consideredto cause the 1984 western Nagano prefecture earthquake.The objective of the survey was to delineate theconductivity structure of the seismogenic region. Aresistivity section of 2-D inversion applied to MT datawere compared with that of joint 2-D inversion applied to CSAMT and MT data.Two resistive anomalies and one conductive anomaly weredetected in regions of intermediate conductivity(100~1,000 ohm-m) in the profile. The resistiveanomalies are considered to he corresponding to thegranite intrusive rock. The conductive anomaly islocated on the hanging wall side of the estimatedearthquake fault and aftershock activity was low inthis area. It is considered that there was many cracksthat were filled with water and slipped like creepingin the conductive zone, and therefore a big destructiondid not occur and aftershock activity was low.The MT data in conjunction with the CSAMT data areverified to he very important to understand the deepgeological structure which is associated with the faultsystem and earthquake of the study area.